The present invention relates to an ignition system for an internal combustion engine which operates with a capacitor which is charged by a magneto. More particularly, the invention relates to an ignition system for an internal combustion engine in which the ignition timing is maintained substantially unchanged, irrespective of the speed of the engine.
In general, in an ignition system using a magnetic generator as its power source, the ignition timing advances with the speed of the engine because the output of the magneto, which is turned by the engine, increases as the speed of the engine increases. Advancement of the ignition timing is suitable for an internal combustion engine used in a garden tractor or the like, but it is not suitable for an outboard internal combustion engine in which the ignition timing is advanced by turning the stator of the magneto in association with the throttle. That is, the position of the stator of the magnetic generator should be determined according to the degree of opening of the throttle for best results. Therefore, if the ignition system has an ignition timing advance characteristic, the ignition timing may be shifted from the best position. Because of this, in an ignition system used in an outboard internal combustion engine, it is required that the ignition timing be maintained constant, irrespective of the speed of rotation of the engine.
On the other hand, in order to eliminate electromagnetic noise produced by the magneto, it has been known to connect a capacitor between the gate and the cathode of the thyristor rectifying the output of the magneto. The noise component may be eliminated more effectively by high values. However, if the capacitance value is too high, when the engine rotates at high speeds, the capacitor will not be completely discharged for every period, and the effect of eliminating noise is lowered. In addition, as the thyristor is continuously triggered by the voltage remaining in the capacitor, the output of the generating coil will be short circuited by the thyristor, and accordingly no ignition voltage produced. Because of such a limitation, heretofore the capacitance of the noise eliminating capacitor is typically limited to 0.047 .mu.F to 0.1 .mu.F. Thus the conventional method is insufficient to effectively eliminate electromagnetic noise, especially low-frequency, high-level noise.